The development of D. (Didymograptus) murchisoni (Beck), the type species of the genus, is shown to be unequivocally of anus type, with thl1 dicalyal and a low metasicular origin of thl1. An evolutionary line from the early, Arenig, bifidus group is supported; and clear distinctions are drawn between the subgenera D. (Didymograptus), D. (Didymograptellus) and D. (Jenkinsograptus), the first of these being redefined. The range of variation of biocharacters of D. murchisoni is analysed using exquisitely preserved undeformed material from Saudi Arabia, and comparisons are made with the undeformed type material, and with undeformed (nodule) collections from Abereiddy Bay, Pembrokeshire. The astogenetically late developments of D. murchisoni are described.

D. murchisoni (Beck 1839), the types species of Didymograptus M'Coy, has been the subject of considerable confusion as a result of the extreme taxonomic splitting within the didymograptids. An attempt is made here to show the range of variation in this graptolite, based on the population on the original type slab, and to suggest ways in which studies on populations can lead to better understanding of the stratigraphic range of these Ordovician forms. Gruptolithina, Didymograptus murchisoni, taxonomy.

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